The present invention relates to an electronically controlled, mechanical timepiece and a control method for the timepiece, in which mechanical energy in a mechanical energy source such as a mainspring is converted into electric energy by a generator, a rotation controller is driven by the electric energy to control the rotation period of the generator, and a hand attached to a train wheel is thereby accurately driven.
Japanese Examined Patent Publication No. 7-119812 and Japanese Unexamined Patent Publication No. 8-50186 disclose electronically controlled, mechanical timepieces that present accurate time by driving accurately hands attached onto train wheels. In such watches, a mainspring, when unwound, releases mechanical energy, which is converted into electrical energy by a generator. The electrical energy is then used to drive a rotation controller so that the current flowing through a coil of the generator is controlled.
The watch disclosed in Japanese Examined Patent Publication No. 7-119812 features two angular ranges: an angular range in which a brake is turned off each time a rotor makes every turn (namely, for each period of a reference signal) to heighten the rotational speed of a rotor so that the generated power is increased, and an angular range in which the rotor is turned at a low speed with the brake applied. The efficiency in power generation is increased during a high-speed rotation to compensate for a drop in power generation that takes place during the braking period.
In the watch disclosed in Japanese Unexamined Patent Publication No. 8-50186, a reference pulse and a measurement pulse detected in the course of rotation of a rotor are counted. The numbers of reference pulses and measurement pulses are compared with each other. In a first state in which the number of reference pulses is smaller than the number of measurement pulses, a controller generates a brake signal for brake control, the width of which is determined by the measurement pulse.
In either of the previously described electronically-controlled, mechanical timepieces, torque (mechanical energy) applied by a mainspring to a generator is set such that a hand is turned at a speed faster than a reference speed, and the rotational speed of the hand is adjusted by applying a brake through a rotation controller.
The watch disclosed in Japanese Examined Patent Publication No. 7-119812 performs brake-activation control and brake-deactivation control for each rotation of the rotor; namely, every reference signal. When the generator initially starts up or when the generator is largely out of control, however, the rotational control amount applied to the rotor cannot be set large enough for every reference signal to adjust the speed of the rotor in a timely fashion. Thus, a long period is required before the watch reaches its normal control state.
In the watch disclosed in Japanese Unexamined Patent Publication No. 8-50186, the pulse width of the brake signal generated for each reference signal is constant. Even with the watch largely out of control, the amount of braking for each reference signal remains constant. The watch thus needs a long period of time before reaching its normal control state.
In addition to a circuit for detecting first and second states by comparing the counts of the reference pulses and measurement pulses, a controller is required to generate a brake signal having a pulse width determined in response to the measurement pulse. Such an arrangement requires a complicated construction, which increases the cost of the watch.
In an electronically-controlled, mechanical timepiece, when the torque of the generator becomes insufficient due to the unwinding of the mainspring and the corresponding weakening of its spring force, the number of revolutions of the generator drops, thereby lowering the speed of a hand, which causes the watch to continuously lose time for a long period of time. Since the hand continuously moves even at a slow speed in this case, a user may glance at the watch and may be under the mistaken impression that the watch works correctly, even if the watch presents an incorrect time.
Accordingly, it is desirable to provide a timepiece that overcomes the drawbacks of the prior art.